Section U-1 of the ASME code lists certain types of pressure vessels that do not require a code stamp. For example, any pressure vessel that has an internal or external pressure not exceeding 15 psi, such as an atmospheric storage vessel, does not require a stamp. Some types of vessels do not require stamps because they are covered under other codes, such as pressure vessels for human occupancy (things like submersibles and hyperbaric chambers), piping system components, boilers, and hot water heaters. Vessels of small diameter (less than 6”) are also exempt. If a code stamp is not required, then the vessel does not need to be designed or fabricated per the rules of the code. Good engineering practice and judgment should still be used to design any non-code vessel or tank to ensure safe and reliable operation.